Studies were undertaken to determine the prevalence and natural behavior of type C RNA tumor viruses of cat, mouse, and man. Humans in close contact with cats with positive evidence of leukemia virus infection were found to have no demonstrable virus neutralizing antibodies against the three known envelope antigenic types of feline leukemia virus (FeLV). Envelope protein of woolly monkey sarcoma virus (WMSV) was found to bind to human, hamster, and mouse cells with same kinetics indicating the presence of viral receptors on these cells. Hybrid cell lines were developed by fusion of Abelson leukemia virus producing cells and Chinese hamster cells to map the integration site of this virus in mouse chromosomes. Similarily in an attempt to map the integration site of WMSV and FeLV in human chromosomes somatic cell hybrids were developed by fusion of human cells infected with appropriate virus and Chinese hamster cells. The loss of virus expression in these hybrid cells is being correlated with the loss of a particular chromosome. Hybrid cells have also been developed between human myelogenous leukemic cells and CH cells to map the genes coding for terminal transferase activity.